Add WebGPU stubs for chat example

[bmorphism]

Summary

• introduce simulflow-examples.webgpu-processors with HTTP proxies for the local WebGPU LLM and TTS endpoints
• allow the text chat example and CLI (bin/chat) to opt into the WebGPU processors via :llm/provider :webgpu / --webgpu
• surface service location metadata and convert float32 audio payloads into PCM16 frames for playback

Testing

• clojure -M:with-examples -m simulflow-examples.text-chat --webgpu